It is sometimes desired to permit the remote control operation of machinery. For example, U.S. Pat. No. 3,793,736 discloses apparatus for establishing a non-conductive data link between command and response locations. Thus, for example, an operator could command certain operations to be performed on overhead high-voltage lines and cables, without being directly in a conductive path.
In other applications, such as the remote operation of mining equipment, the operator may wish to place himself in a location removed from an environmental hazard.
In any event, in the remote operation of machinery, it is imperative that the machinery respond only to the intended command signal, and be non-responsive to any extraneous signal even if generated on the same communication path (e.g., frequency or data bus). In other words, cross-coupling should be prevented.